THE ROOFTOP POWER REVOLUTION

From issue 2535 of New Scientist magazine, 21 January 2006, page 36

Mick Hamer

LATER this year, when Queen Elizabeth turns 80, she will see a dramatic
drop in her electricity bills. This is not a special deal for the UK's
senior citizens. The cut in the royal fuel bill will come thanks to a
hydroelectric plant on the river Thames, which will start supplying power to
Windsor Castle, one of her numerous homes.

The £1 million scheme, which will supply up to 200 kilowatts of
electricity, is being developed by the electricity generating company
Npower. The generators will be driven by four turbines now being built into
Romney weir, about 800 metres from the castle. The generators will make a
substantial dent in the royal electricity bill, supplying about a third of
the castle's needs. And the cost of the upkeep will be minimal. "Once
they're in you can pretty much leave them alone," says Patrick Spink of
Npower Renewables, the company installing the turbines. "They are likely to
be there for 100 years."

Not everyone has a large river at the bottom of their garden. But almost
all of us can trim our utility bills by generating our own energy.
Photovoltaic tiles or a small wind turbine on the roofs of houses or
apartment blocks are no longer a rarity. If these and similar small-scale
generators were installed in large numbers they could have a significant
impact on energy policy, helping to slash carbon emissions and taking the
strain off overloaded distribution grids. A growing enthusiasm for renewable
energy has also stimulated development of new small-scale energy generators
that are reliable, simpler to install and, most importantly, capable of
exporting the power they create onto the grid. "The potential is pretty
significant," says Dave Sowden of the Micropower Council, the UK industry
association that promotes small-scale power generation. "We are talking
about turning power generation into consumer products that you can buy at a
DIY store."

Faced with record oil prices and a desire to cut carbon emissions, most
governments are looking at alternatives to fossil fuels. But can converting
homes into power stations really turn conventional energy policy on its
head? How much power can microgeneration contribute, and what technical
issues must be solved before we can all begin to harvest and sell our own
energy?

A single 1.5-kilowatt wind turbine seems insignificant compared to a
1000-megawatt nuclear power station, which some tout as the only hope of
curbing greenhouse emissions. But according to a recent UK report prepared
for the government-sponsored Energy Saving Trust (EST), home power
generators of various kinds could provide 30 to 40 per cent of the country's
electricity needs by 2050 - by comparison the UK's nuclear industry provides
20 per cent of current needs.

Microgeneration can bring several advantages. Energy from wind, water or
the sun does not depend on gas or oil from countries whose governments can
turn off the tap without warning or hike up the price. On top of that, the
EST estimates that by 2050, microgeneration could cut the UK's carbon
emissions by 15 per cent compared with the present mix of energy generation.
And unlike large generating stations, which require a lead time of years,
microgeneration capacity can be built up steadily - an incremental change
that could, for example, remove the need for a new nuclear power programme.

The UK's liberalised energy market has helped to make it one of the
world's leaders in microgeneration, says Jon Slowe of Delta Energy &
Environment, a European consultancy specialising in small-scale power
generation. The UK also has a microgeneration strategy document due for
delivery this spring. "It's quite a contrast to the rest of the world,"
Slowe says. "No one has really paid any attention to the household sector
before."

So what qualifies as a microgenerator? In the UK the term is used to
describe generators with an output of less than 50 kilowatts, but in other
countries the term small-scale can apply to installations running into the
megawatt range. And there is no single favoured way of generating power
(see "Home-grown power"). Manufacturers of
wind turbines and photovoltaic panels say global sales are growing by 20 to
40 per cent a year. Water power is gaining popularity too: in the south-west
of England, for example, dozens of old water mills are being equipped with
turbines capable of generating tens of kilowatts. Water wheels have
returned, with a new generation of designs from German and US companies.

Cut out power cuts

Even the Archimedes screw has been transformed into a microgenerator,
thanks to Ritz-Atro, a company based in Nuremberg, Germany. Its devices are
capable of generating anything from 1 kilowatt upwards, from flows of just
70 litres per second. The company says they could be used in situations
where the flow is too low for other microgenerators, such as small weirs or
the waste outflows from small industrial plants. Two small screw generators
have been installed in outflow pipes at water treatment plants near Tübingen
in Germany, for instance.

A study last year by Amory Lovins of the Rocky Mountain Institute in
Snowmass, Colorado, showed that the output of small-scale unconventional
power sources is starting to outstrip that of the world's nuclear power
industry
(see Graph). Even Lovins was
surprised. "I knew things were big but nobody had ever added them up
before," he says.

Part of the attraction of small-scale generation is that it is well
suited to renewable sources of energy such as wind, water and solar. These
can be used in conjunction with micro-combined heat and power (micro-CHP)
units, in which some of the heat output of a gas boiler is used to make
electricity. "One of the beauties of microgeneration is that the
technologies are complementary," says Walt Patterson, an energy specialist
with the London-based think tank Chatham House. "If the sun isn't shining,
it's often windy." Just as usefully, the electrical output of a micro-CHP
unit follows a consumer's demand for heat. So while photovoltaic cells
produce electricity during daylight hours, micro-CHP can kick in when people
come home in the evening and switch on their heating.

Small-scale power generation has other advantages, too. Producing power
locally avoids distribution losses, which account for about 10 per cent of
the electricity fed into the grid. This also helps ensure security of
supply, says Lovins, by avoiding the power cuts that arise when the grid
fails - the main reason for power failures in the US.

According to Mike Bergey, president of Bergey Windpower of Norman,
Oklahoma, after the power cuts that hit California in 2001, sales of the
company's 10-kilowatt wind turbines went up by 400 per cent. "They would
have grown three to four times that if there had not been the huge problem
with getting building permits," he adds.

One problem with small-scale generators is that their power output
fluctuates. If you have a wind turbine on your roof, you'll need electricity
from some other source when the wind dies. And if you're away from home when
it's windy, you'll generate more electricity than you can use. The obvious
answer is to buy in extra power from the grid when needed, and sell power
back to the grid at times when you are generating a surplus. But the
electricity grid was built to distribute power, not receive it. As a result
there are a number of obstacles, both technical and commercial, to doing
this. Power companies in many countries prefer not to let smaller generators
connect to their grids as they fear technical problems could arise. There is
often no system for paying microgenerators either. In the US, for example,
only 34 states have regulations that provide a mechanism for paying people
who supply electricity.

Power companies are frequently obstructive, says Lovins, and may demand
exhaustive engineering studies before allowing a connection. "I think the
basic problem is that most US utilities do not understand that distributed
generation is in their financial interests, so they fight it." Power
companies often fail to see that microgeneration can help them avoid
expensive upgrades to the grid, for instance. As a result, says Lovins, the
US is lagging far behind Europe.

There also needs to be a fair system to pay people for the electricity
they generate. But what price is fair? There is general agreement that
households should be paid less for the power they sell than they are charged
for the power they buy, to cover the cost of installing and maintaining the
distribution network. It's just how much the difference should be that is in
dispute.

Such commercial problems are mirrored by technical ones. Electricity
meters were designed to monitor how much power a household consumes, not how
much it produces. A survey by the UK's energy industry regulator Ofgem in
October 2004 found that about two-thirds of the country's 26 million meters
had a reverse stop, preventing them registering any power fed back into the
grid. Anyone else who connects a generator such as the Windsave wind turbine
(which simply plugs into the mains) will see their electricity meter running
backwards on windy days - dismaying power companies, which expect customers
to help pay the cost of maintaining the network.

The simplest solution would be to wire in a separate meter to measure
electricity exports, so that they can be paid at the appropriate rate.
Alternatively, smart meter systems that use processors and software can
record energy flow accurately, calculate the exact costs of power usage and
automate energy export.

The electricity industry has voiced fears that microgeneration on a large
scale would create unacceptable voltage fluctuations on the distribution
grid. The UK's grid, for instance, consists of networks linked by
transformers: the high-voltage network is used for energy transmission, a
medium-voltage network supplies power-hungry industrial users and a
low-voltage network supplies homes and small businesses. Anything attached
to the network - be it phone charger or electric furnace - creates voltage
fluctuations when it is switched on or off. If large numbers of wind
turbines or solar cells attached to the low-voltage network start to
generate power simultaneously, it is possible they could create voltage
fluctuations capable of damaging equipment or even causing blackouts.

The grid was, after all, designed to transmit electricity from power
stations through substations to the low-voltage cables that connect homes.
It was not designed to receive power from a large number of small sources.

Yet according to a report in 2004 by consultants Mott MacDonald, such
fears are unfounded, in the UK at least. It found that most of the country's
low-voltage networks wouldn't have any problems, even if every home was
capable of generating power. Only on rural networks, where there are lengthy
cable runs, is there expected to be a problem. And even this can be remedied
by reducing the output from the transformers that control the voltages on
these lines, something the report says can be done during normal
maintenance. On the plus side, microgeneration could avoid expensive
upgrades to the distribution grid which would be needed if predicted growth
in demand is met solely by centralised generators.

Most microgenerators are not yet the simple-to-install consumer products
that advocates hope they will become. Micro-CHP must show that it is
reliable, for instance. And the manufacturers of small wind generators will
have to convince the public that they have eliminated the noise that many
people still see as a problem with wind turbines.

Crucially, the prospects for microgeneration depend on how quickly the
cost of installing the equipment can be recouped. Utility bills tend to
track the price of oil, so last year's sharp rise in the oil price should
improve the economics of microgeneration. A micro-CHP system will now pay
for itself in about 12 years, a small wind or water turbine in slightly
less. But it still takes around 50 years for most photovoltaic systems to
pay for themselves.

“The prospects for microgeneration depend on how quickly
the cost of installation can be recouped”

At the root of the problem is the obstacle that often faces new
technologies: production runs for domestic power generators are still small,
so individual units are expensive. The EST report suggests that governments
could help by subsidising the cost to consumers, and introducing a
requirement that all new buildings should include microgenerators. Yet Japan
and the UK have restricted subsidies for renewables and the 2005 US Energy
act contained little encouraging news for microgeneration. If the queen of
England can have her personal power station, shouldn't we all?

Home-grown power

Microgenerators can harness energy from sources ranging from natural gas to
the sun's rays. But in the UK at least, one of the primary sources is wind
power.

Two of the rooftop wind turbines undergoing trials in the UK come from
companies in Scotland: Renewable Devices of Edinburgh and Windsave of Glasgow.
Renewable Devices' Swift turbine produces 1.5 kilowatts in a
10.5-metre-per-second wind. This year the company signed an agreement with the
Perth-based company Scottish and Southern Energy to supply turbines to
customers.

The 1.75-metre Windsave turbine produces 1 kilowatt in winds of 12 metres
per second, and trials of it are being backed by British Gas. Key to the
design is that it simply plugs into a standard mains socket. An inverter
converts the output from the turbine into 230 volts AC, and a sensor adjusts
the phase of the output to match that of the mains. "It's counter-intuitive,"
says Walt Patterson of Chatham House. "You plug it in to generate
electricity." Both turbines are expected to cost about £1 per watt to install
and could pay for themselves in eight years or so.

The outlook is less bright for photovoltaic panels. Global PV capacity is
doubling every two years, but less than a quarter of existing PV installations
are economic, and only pay off where there is no mains electricity. The
Eddystone lighthouse off the coast of south-west England, for example,
replaced diesel generators with photovoltaic cells in 2000.

In Germany and Japan, PV generation has been encouraged by subsidies to
grid-connected households that put solar cells on their roofs, and these two
countries have about two-thirds of the world's PV generating capacity. The
breakthrough will come when the cost of solar electricity matches that of
mains power - most likely a result of mass production rather than improved
efficiency. By 2020 PV electricity should cost roughly the same as the
forecast level of peak electricity tariffs.

Microgeneration is not just about generating electricity: you can make your
own heat too. One option is to extract heat from the ground using a heat pump
that pulls heat out of a mixture of water and antifreeze flowing through a
closed loop of pipes laid in the ground. Ideal for underfloor heating, heat
pump systems are already as cheap as oil-fired central heating. Solar heating
is also widely used to provide hot water - and not just in sunny locations
like the Mediterranean and southern California. Some 70,000 houses in the UK
use solar water heating, for instance, and the government estimates that by
2010 some 50,000 households a year could be installing the technology.